This invention pertains to the art of electrical control circuits and more particularly to a solid state control circuit for switching a large inductive load.
The invention is particularly applicable to a simple power control circuit which displays desirable operating characteristics especially for loads whose power factor does not vary a great deal over time or does not vary rapidly. For example, a load such as a channel induction furnace which would have essentially the same power factor for months or which would change very slowly, is the type of load that the subject invention is intended to control. However, it will be appreciated by those skilled in the art that the invention could be adapted for use in other environments as, for example, where similar power circuits are employed to control the application of electrical energy to a load.
In the past, most people have refrained from using solid state controls, such as thyristors, for switching the power to induction furnaces because of the great need for reliability of the control circuit to operate the furnace. The risk of power outages, which were to be expected with solid state controls, was unacceptable for powering an induction furnace where the control circuit must operate reliably.
It has been known to employ solid state controls in a part of the control circuit for an induction furnace; however, the solid state switches have been employed to switch capacitors, and have not been employed to switch the power directly to the furnace for the reasons discussed above. In other words, the control has been essentially achieved by connecting the furnace in series with a capacitor and by rapidly switching capacitors in and out, the power to the furnace could be controlled, for example see U.S. Pat. No. 4,037,044 (Havas). A problem with such circuits is that the switching of capacitors with thyristors is a rather difficult thing to do. If the switches are not turned on at the proper time they will self-destruct. Consequently a small error in switching time can have disastrous consequences in the failure of the control circuit. Despite this risk, the prior art systems employed a plurality of thyristors switching an associated plurality of capacitors to effect the rapid switching of the capacitance of the control circuit. The rapid switching, sometimes at a switch per half cycle, required an extremely complicated control circuit which is expensive to produce and maintain.
Another problem with the prior art systems which include thyristors switching a plurality of capacitors is the difficulty such control circuits have in minimizing the potential harmonic resonances in the system. Any such control circuit must constantly be aware of the effect of resonant frequencies which are picked up on the line side or which are generated by the control circuit itself.
Yet another design problem for such induction furnace control circuits is the ability of the circuit to handle shorts. Due to the high power levels involved in powering an induction furnace, any control circuit must be designed with a consideration of the consequence of a short and, accordingly, the elements must be designed to handle a much larger current than they are intended to handle during normal operation. Again, this will involve an undesirable increased expense.
The present invention contemplates a new and improved control circuit for switching power to an induction furnace which is simple in design, economical to manufacture, readily adaptable to a variety of furnace loads, which provides improved filtering of potential harmonic resonances, and is self-limiting current wise if the load is short circuited.